1. Field of the Invention
The present invention relates to a photoacoustic apparatus which obtains information on a specimen by receiving photoacoustic waves generated from the specimen resulting from light irradiated to the specimen, and it also relates to a probe for receiving the photoacoustic waves.
2. Description of the Related Art
As one of the optical imaging techniques, there has been proposed photoacoustic tomography (PAT) (see a first non-patent document). The photoacoustic tomography is a technique in which a specimen is irradiated with pulsed light generated from alight source, and acoustic waves are detected which are generated from a living body tissue that has absorbed the energy of the light propagated and diffused in the interior of the specimen, whereby those acoustic waves or signals are subjected to analytical processing to visualize the information relevant to optical property values in the interior of the specimen. That is, a photoacoustic effect is a phenomenon in which when pulsed light is illuminated on a material or specimen, an ultrasonic wave (compression wave) is generated due to the cubical expansion of the specimen in a region thereof where the optical absorption coefficient thereof is high. The ultrasonic wave generated due to the cubical expansion of the specimen by irradiating the specimen with pulsed light is called a “photoacoustic wave” in the present invention.
With the above-mentioned technique, an optical property value distribution, especially an optical energy absorption density distribution, in the specimen can be obtained. By using such photoacoustic imaging, an optical property value distribution with a high resolution is obtained.
On the other hand, according to the first non-patent document, it is described that when light is irradiated to a probe surface, a large noise signal will arise which is unrelated to an acoustic signal from the specimen, so an aluminum film, which does not influence the propagation of an ultrasonic wave, is arranged on a front face of a linear array type ultrasonic probe.
[Patent Document 1]    Japanese patent application laid-open No. 2006-208050
[Non-patent Document 1]    Joel J. Niederhauser, et. al., “Combined Ultrasound and Optoacoustic System for Real-Time High-Contrast Vascular Imaging in Vivo”, IEEE TRANSACTION ON MEDICAL IMAGING, Vol. 24, No. 4 April 2005
In photoacoustic imaging, there is a problem that it is difficult to obtain information on a deep portion of a specimen because of the attenuation of light due to the optical absorption in a living body. That is, the sound pressure of the acoustic wave obtained is proportional to the amount of local light which reaches an absorber in the specimen. However, the light irradiated to the specimen is rapidly attenuated in the specimen due to the dispersion and absorption thereof. Furthermore, in the case of a human body, a safety restriction on the amount of light allowed to be irradiated thereto is provided by standards, so it is difficult to provide a large amount of light to a deep tissue in the living body. Therefore, it becomes difficult to obtain photoacoustic waves from a deep portion of the living body.
Consequently, according to the extensive study of the present inventors, it has been found that light is strongly scattered in the specimen, so a part of supplied energy is radiated or emitted out of the specimen while not contributing to the generation of a photoacoustic wave. It is considered possible to suppress the loss of the supplied optical energy by reflecting again the diffused light, which has been radiated out of the specimen without being absorbed therein, in a direction of the specimen. Therefore, if effective use of such lost light can be performed, there must be a benefit that information on the deep portion in the living body can be pictured or imaged, or a small-sized light source can be used.
Incidentally, the first patent document includes a suggestion that light is intended to be used effectively by using a light reflection member in the form of a spheroid. In this case, light is irradiated in an effective manner by being converged to the focus of the spheroid before the light is irradiated to the specimen. However, this is not effective use of the diffused light which is the light radiated out of the living body after entering the specimen. In addition, in this case, it is necessary that the specimen should be located at the focus of the spheroid, and a large probe such as an array probe or the like interrupts the light and hence can not be used. Furthermore, a signal is also unable to be detected while scanning the probe. Thus, since large restrictions arise on the use of the technique of this document, the application thereof to general-purpose imaging apparatuses is difficult.
On the other hand, the first non-patent document describes that an aluminum film having such a thickness as not to influence the propagation of ultrasonic waves is arranged between a linear array type ultrasonic probe and a coupling pad made of agar (i.e., gelatin made from seaweed). However, no detailed sizes thereof are indicated. In general, the width of a linear array type ultrasonic probe is about 1 cm and the width of a receiving element thereof is about 5 mm, so the width of the reflecting film arranged on a front face of the probe is at most about 1 cm. In addition, generally, the linear array type ultrasonic probe is provided with an acoustic lens, and takes a convex shape.
According to the study of the present inventors, though details will be mentioned later, even if there was such a reflecting layer or film of a size thus taken so as to be arranged for the purpose of noise removal, the amount of light reflected in the direction of a living body was not necessarily large. In addition, light diffuses when reflected on the plane of reflection of a convex shape, so the energy density of the reflected light irradiated to the living body becomes still smaller. That is, the construction of the first non-patent document can not utilize the optical energy so effectively, and it is formed only for the purpose of noise removal.
As described above, it is considered that a photoacoustic signal from a living body deep portion can be observed by making effective use of the light radiated out of the living body, but any detailed solution to that has not yet been established.